I have an HP Scanjet 3970 and this is a frequent problem.
I simply reach behind the scanner, unplug the power connector, wait 15 seconds then plug it back in. It makes a little grunting noise and the lamp turns on and it works just fine for a few days until it locks up again.
This "reset's" it and clears whatever's stuck in it's craw.
P.S. This also works on Routers and Cable Modems and almost any external perepherial that locks up.
~ Raven ~